Thermally induced chemical changes in the macromolecular structure of an Indonesian coal

A short time variable temperature heat treatment in moist air at 250°C has been applied to an Indonesian sub-bituminous coal to produce a thermally stabilised coal product and to effect an irreversible lowering of the Equilibrium Moisture Content. Elemental analyses show that the uptake of oxygen is less than expected for an oxidative fixed temperature heat treatment over the same time period and sulphur is significantly reduced in concentration. Pyrograms at 340 and 600°C show that the heat treatment raises the pyrolytic yield at 600°C whilst reducing it at 340°C. DRIFT and 13C SS MAS NMR spectroscopy have been used to follow chemical changes. Oxygen uptake appears to occur at bridging and other sp2 carbons to yield substituted aromatics including phenols. These phenolic groups can react to form esters and ethers. In general, the functional groups observed are those conventionally described for coal oxidation but differ significantly in the thermally stabilised coal in that there is a reduced concentration of carbonyl groups; attack on aliphatic carbons is less; and oxygen substitution is emphasised.